Mailvelope

Mailvelope is an open source browser extension for Google
Chrome and Mozilla Firefox that allows the exchange of
encrypted emails following the OpenPGP encryption standard.
Mailvelope instantly secures your personal and professional email
communications.

Mailvelope adds the controls to encrypt and decrypt emails
directly to your Webmail provider. It can be configured for any
provider.

Mailvelope is based on OpenPGP.js, a JavaScript implementation
of the OpenPGP standard.